Bluetooth: Controller: Only validate CIS SDU interval for BN > 0

Some in-the-wild devices use SDU interval of 0 when BN == 0; This
is not allowed by BT Core Spec v6.0, but is not specifically
mentioned in v5.4 and earlier. To allow connecting a CIS to these
devices, relax the check on SDU interval

Note: LL/CIS/PER/BI-07-C only checks rejection of invalid SDU interval
with BN > 0, so it still passes with this change

Signed-off-by: Troels Nilsson <trnn@demant.com>